Various imaging techniques, such as X-rays, fluoroscopy, ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI) play an integral role in a wide variety of medical procedures. The term “image assisted” has been adopted to distinguish these procedures that are performed through the use of some type of imaging-based systems.
The incorporation of image guidance systems into various procedures allows a physician to correlate a desired location on a patient's anatomy to images taken pre-operatively or intra-operatively using various imaging modalities such as x-rays, ultrasounds, CT scans, or MRI's. The use of image guidance systems also imparts the ability to look through superficial layers of anatomy to visualize deeper targets of interest. Further, image guidance systems provide the guidance needed to access target areas of interest within the patient's anatomy through the use of pre-defined entry and/or target zones. Often, physicians rely heavily on imaging systems when a target cannot be directly visualized in order to avoid damage to surrounding anatomical structures and to minimize unnecessary tissue trauma.
There are at least two “spaces” used in image guidance systems. The first is the “image space,” which is the imaging acquired prior to or during a procedure, such as an MRI scan of a specific anatomical area done before surgery. From cross-sectional imaging, a three-dimensional data set may be constructed using the first image space's coordinate system, usually expressed as a Cartesian system with an arbitrary origin and principle axis. The second space is the actual physical space surrounding the patient. This is often restricted to a specific anatomical part, such as the head, lower back, hip joint, etc., in order to improve local resolution and system performance. An image guidance system may include a mechanism for accurately measuring position within the patient's physical space, much like a tracking device. The tracking device may have its own coordinate system different from that of the “image space.” In order to provide flexibility, there is often a “reference” that is held in a rigid relationship relative to the patient's anatomical area of interest. The reference serves as an arbitrary origin of the patient's physical space and all three-dimensional spatial measurements made can be expressed relative to this reference. The use of a reference allows for the movement of the image guidance system or for the manipulation of the target anatomical region without losing registration or affecting guidance accuracy.
After the two coordinate systems have been established, the image space may be correlated to the physical space through a process known as registration. Registration refers to the coordinate transformation of one space into another. This is usually a linear and rigid transformation in which only translation and rotation takes place (no scaling and no local deformation).
Once registration is completed, a probe or other device may be used to touch various anatomical structures on the subject (physical space), and the corresponding images of the same anatomical structures may be displayed (image space). The image guidance system may have the added advantage of multi-planar reconstruction, which allows the three-dimensional image dataset to be displayed in any arbitrary plane, further allowing users to view the surrounding structures through any arbitrary direction.
An image guidance system may include an information processing unit (e.g. a computer), which is used to load a patient's pre- or intra-operative images, as well as to run the software that will perform the registration between the image space and the physical space. The software program performs the registration between image space and physical space, and provides navigational information to the operator. This often includes the ability to perform multi-planar reconstructions and to perform targeting with specification of entry and target zones. More advanced functions include image fusion capabilities across imaging modalities such as fusing CT imaging data with MRI imaging data, as well as advanced image segmentation (e.g. extracting image information of a tumor or vessels and rendering three-dimensional models of these structures) to provide surgeons with live intraoperative guidance.
Another component of an image guidance system is the tracking device or reference that is used for spatial recognition. This device reads the coordinates of any point in three-dimensional space to allow accurate tracking of the physical space around the patient. An image guidance system also may include various probes to allow tracking of instruments, such as surgical instruments, endoscopic tools, biopsy needles, etc., during operation to provide flexibility with regards to navigational options. The probe may also act as the tracking device or reference.
Based on the aforementioned concepts, various advancements have been made that resulted in the inception of various image guidance systems. These systems differ on the exact detail of their execution regarding the various system components; however, many commonalties exist between the systems.
The most common system for spatial navigation is an optical system, such as that disclosed in U.S. Pat. No. 5,230,623. An optical system includes a stereo camera (i.e. two cameras mounted a known fixed distance apart) that cooperate to provide accurate three-dimensional localization. The method of tracking can be either passive or active. In passive tracking, the system emits infrared radiation (usually through a ring of infrared light emitting diodes, or LED's, mounted around each camera) and passive optical markers reflect the radiation back to the camera and allow the markers to be seen. The markers are usually small spheres of a pre-defined diameter coated in a reflective coating optimized for the wavelength of infrared radiation. With active tracking, the markers themselves consist of infrared LED's which emit infrared radiation that can be directly seen by the camera. Three or more markers arranged in a predefined geometry can be used to give total specification of a unique vector with 6 degrees of freedom (DOF)—3 in translation and 3 in rotation. By altering the predefined geometry of the markers, the system can recognize and simultaneously track various probes and tools, including the special “reference probe” that defines the arbitrary origin in the physical space. Optical systems typically come with proprietary software that performs image registration and provides navigational information to the end user.
Another system for spatial navigation is a magnetic system, such as the AxiEM™ navigation system marketed by Medtronic. A magnetic system employs a magnetic field generator to generate a uniform gradient field. A sensor is used to measure the strength and direction of the magnetic field, and based on this information, spatial localization is derived. Again, a reference point is fixed to the patient and various probes are available for flexible navigation.
Another system for surgical guidance is a stereotactic system. For cranial procedures, these systems rely upon the attachment of a rigid frame around a patient's head. Cross-sectional imaging (CT or MRI) may then be taken of the patient's head with the frame attached. The frame provides measurement of the physical space around the patient's head and correlates directly with the image space since the frame is captured on the scan. Registration of the image space and physical space occurs automatically once a common arbitrary coordinate system is chosen on the scan. Guidance is achieved mechanically, meaning that an external mechanism usually directs the surgeon's instrument down a machined groove or bore. The surgeon must rely solely on the trajectory calculations since no visual feedback is available in the absence of real-time imaging (e.g. intra-operative CT or MRI scanning).
Mechanical guidance can be expressed in various coordinate systems—Cartesian, polar, spherical, or mixed. The Leksell Stereotactic System® marketed by Eleckta is a common stereotactic system in use today, and it uses a mixed system. It expresses the target in Cartesian coordinates of x, y and z. The mechanical guide relies on the “arc” principle, whereby the arc is always centered over the target. This allows the surgeon to pick any ring or arc angle to find the most optimal placement of an entry site. Alternatively, an entry site could be predefined and the arc/ring angles could be calculated. Various size guides are available to accommodate various instrument diameters. Since the system cannot provide live image guidance, its role is more limited to procedures such as biopsies or placement of electrodes. A more specialized application of the Leksell frame is encountered in gamma knife therapy to help localize the radiation target. Numerous other stereotactic frames are currently available on the market that essentially embody various iterations of the same underlying principle.
Image navigation has proven to be extremely useful in improving accuracy of targeting, avoiding damage to surrounding critical structures, and improving patient outcomes. However, accurate targeting of deep anatomical structures is challenging across multiple disciplines. There is a need for an image guidance system which facilitates identification of ideal trajectories that are not directly visualizable.
Several clinical applications would stand to benefit from such improved targeting methods. One example is the insertion of external ventricular drains (EVD) or ventricular shunts (ventricular peritoneal, ventricular atrial, ventricular pleural, etc.). This procedure is performed to release/redirect cerebrospinal fluid (CSF) and to monitor intracranial pressure (ICP). The current standard of care involves a blind passage of the ventricular catheter from the skin surface to the deep ventricular system in the brain via crude external landmarks. Current image guided systems used in this procedure rely upon rigid fixation of the head and access to the operating room. In addition, the use of existing image guided systems may significantly lengthen the procedure time, making their use in the emergency setting unsuitable, especially when urgent control of ICP is needed.
Another clinical application that could benefit from improved targeting methods is the performance of biopsies and related procedures. Accurate targeting of soft tissue, bone, fluid, or anatomical spaces may be used to facilitate biopsy, device placement, and/or pharmacological agent delivery. A common cranial application is a stereotactic biopsy. Traditional methods have focused on frame-based stereotactic biopsy that relies upon the application of a frame secured to the skull with sharp pins that penetrate the outer table of the skull (e.g. four pins for the Leksell Stereotactic System® marketed by Eleckta). This procedure is painful for the awake patient and cumbersome to set up. Recent advancements in image guidance systems have allowed the development of “frameless stereotaxy.” In this instance, the pre-procedural application of a frame followed by imaging of the patient with his/her head in the frame is avoided. However, the head still needs to be rigidly fixed with penetrating pins in a skull clamp, such as the Mayfield® clamp marketed by Integra LifeSciences. Because of the pain of fixating the skull and the immobilization experienced with the 3-pinned Mayfield® system, the patients are typically given a general anesthetic. With frameless stereotaxy, the targeting information is shifted entirely to the guidance system and the screen. The surgeon may unfortunately need to periodically look away from his or her hands and surgical instruments to view a screen that helps guide the trajectory.
Similar systems have been deployed to place electrodes or other implants. For instance, deep brain stimulator or RF ablation electrode insertion into cranial structures employs similar steps as a stereotactic biopsy. In this instance, the goal is to place an implant into a pre-defined area of the brain. Again, utilizing similar image-guided techniques, abnormal fluid or soft tissue collections including, but not limited to intracerebral abscesses, hematomas, or protein collections can be accurately targeted.
There are numerous potential applications of image-guided techniques in orthopedic procedures, ranging from placement of implants to placement of nails, plates and screws. For example, in hip replacement surgeries, accurate placement of the acetabular cap with specific angles of abduction/adduction and flexion/extension has been shown to be an important factor in preventing premature wear and recurrent hip dislocations. Similarly, knee, shoulder, ankle and small joint replacements rely upon precise cuts in the adjacent bones to ensure anatomical alignment of the implant. Another example is the placement of pedicle screws in spinal surgery, which rely upon a precise trajectory and angle of insertion to prevent neurological injury and screw misplacement. Another frequent orthopedic application involves the placement of intramedullary nails in long bone fractures. Intramedullary nails may conform to the shape of the intramedullary canal, sometimes making accurate targeting and alignment of distal locking screw holes difficult. Unfortunately, although many attempts have been made, no satisfactory system yet exists that can easily address this problem without significantly lengthening the operative time.
Unfortunately, all of these techniques, whether major or minor, involve access to an image guidance system, a fixation method, and an operating room. Access to such facilities and instruments may not be feasible if an emergency procedure is needed, where the delay in bringing the patient to the operating room and setting up existing image guidance systems would result in catastrophic outcome for the patient. The physician is often forced to resort to crude external anatomical landmarks for guidance. This trade-off between speed and accuracy means that patients who require emergency procedures are often not able to receive the benefits of image-guidance. Further, existing image guidance systems are, in many instances, expensive and cost-prohibitive in smaller medical facilities. This restricts access to image guidance technology to large, well-funded hospitals. Many hospitals and healthcare facilities are not equipped with traditional image guidance systems, depriving patients of the benefits of the accuracy and precision of image-guided procedures. This is particularly true in developing countries where cost is a major barrier to the adoption of image guidance technology. Additionally, routine radiology procedures such as biopsies are performed under the guidance of plain films, CT scans, ultrasound imaging, and magnetic resonance imaging. All of these imaging modalities require the practitioner to view an image on a screen, computer terminal, or the like, instead of watching the procedure in the physical space. These procedures are performed frequently and may expose radiologists and technicians to potentially harmful doses of radiation. When using existing image guidance systems, the users must take their eyes off the patient and focus on the information displayed on the screen (“eyes off target”). For these critical moments, the users do not have direct visual confirmation of their instrument(s). Instead they must rely on feel, muscle memory, and/or rapidly looking back and forth between the screen and the patient. Therefore, a need exists for an image guidance system that can use previous imaging studies to guide the physician as they target a structure hidden below the surface of the skin without the use of frames or pins while providing direct visualization within the working area of the targeting trajectory (“eyes on target”).